wayland: Use correct bounds size when computing the source box

The buffer transform specifies a transform from the buffer coordinate
space to the surface coordinate space.

The inverse buffer transform specifies a transform from the surface
coordinate space to the buffer coordinate space.

OutputTransform::map(QRect, QSizeF) expects both arguments to be in the
same coordinate space.

In case of SurfaceInterfacePrivate::computeSourceBox(), both should be
scaled surface coordinates so bufferTransform.inverted() maps the source
rect to the proper buffer coordinate space.